Hammer firing circuit for high speed printer



Aug. 22, 1961 F. H. SHEPARD, JR 2,997,632

HAMMER FIRING CIRCUIT FOR HIGH SPEED PRINTER Filed Sept. 12, 1958INVENTOR fkmw; flap/1 90 JR.

AT ORN nite 2,997,fi32 Patented Aug. 22, 1961 2,997,632 HAMMER FIRINGCIRCUIT FOR HIGH SPEED PRINTER Francis H. Shepard, Jr., Lee Lane,Berkley Heights, NJ. Filed Sept. 12, 1958, Ser. No. 760,590 5 Claims.(Cl. 317-1485) This invention relates to an improved electric circuitfor firing a type-hammer in a high speed printer, such as the printerdescribed in the inventors US. Patent No. 2,787,210.

An object of this invention is to provide a hammer firing circuit whichcan be made at lower cost than previously known circuits and which willoperate with greater reliability and efiiciency.

A more specific object is to provide such a firing circuit which can bemade more compactly and have greatly increased service life, togetherwith increased ruggedness and reliability.

These and other objects will in part be understood from and in partpointed out in the description given hereinafter.

In a high speed printer of the kind described in the above-identifiedpatent, a strip of paper is passed through the machine and printed uponby blows from an array of hammers positioned beneath the paper and firedup- Ward to drive the paper against corresponding ones of a like arrayof type wheels. These type wheels are mounted side-by-side in the formof a cylindrical drum and carry around their periphery alpha-numericcharacters to be printed on the paper. The type wheel assembly isrotated at high speed, for example 1200 rpm, and the paper is movedstep-by-step at high speed, for example twenty lines a second, so thatthe printing operation can be carried out at a very rapid rate. Eachhammer is in the form of a long, thin rod which is fired end firstupward against the paper to press it against a selected one of thecharacters around the rim of the corresponding type wheel. The time ofprinting is very short, of the order of fifty microseconds, so that eventhough the type wheel is continuously rotating its movement iseffectively frozen during each hammer blow.

In this machine each hammer is propelled against the paper by anelectromagnet which is energized at the proper instant by a firingcircuit electronically controlled to print any desired character. Eachhammer, and there may be for example 190 hammers in a machine, isaccompanied by an electromagnet to fire it and by a corresponding firingcircuit to energize the electromagnet. Thus in a single printer a vastnumber of electric circuits for firing the hammers is required.Accordingly, it is very desirable to have these circuits as inexpensiveand as compact and reliable as possible. The present inventionaccomplishes these objectives.

In a high speed printer of this kind, the firing circuit for eachtype-hammer must be able to energize the coil or solenoid of thehammer-firing electromagnet with a carefully regulated current. Thiscurrent must suddenly and accurately be applied to the coil in order tofire the hammer at the precise instant required for it to drive thepaper against a particular type character of the continuously rotatingtype wheel. Moreover, the energy applied to the coil must not be toogreat nor too small, otherwise the hammer will print with too much ortoo little energy. Since a printer may be operated many hours a day fordays-on-end, billions of hammer blows will be made during the life ofthe unit and this demands an extremely high degree of reliability in theelectronic circuits which time and fire the hammer.

Previous circuits which have been used throughout the industry to firehammers in a printer of this general kind employ a relatively largestorage capacitor which is charged to a high voltage, for example 600volts, and then suddenly discharged by means of a thyratron into thecoil of its hammer-firing electromagnet. Not only does a circuit of thiskind require relatively large stand by power and dangerous high voltagesbut it is relatively inefficient in its operation and poor inreliability. The service life of the thyratron tube in this heavy dutyapplication is limited, and because of the overall inefficiency in theway this circuit operates it is expensive in terms of size of componentsand in power consumed. The present invention provides a much simpler andfar more effective circuit for energizing the hammer-firingelectromagnets.

l n accordance with the present invention a power transistor is operatedas a switch at low voltage and very high current to energize anassociated hammer-firing electromagnet. This transistor is operated andcontrolled by several other transistors to turn on at precisely thedesired instant and for a carefully regulated time. is operated in suchfashion that it is able to handle powers far in excess of its normalrating as an amplifier. By using an all-transistor switching arrangementof this kind, very high speed operation yet extremely precise firingcontrol is obtained and there is no need of storage capacitors whichmight otherwise tend to cause oscillations in conjunction with thehammer electromagnet. By doing away with these storage capacitors, thedelay in recharging them is eliminated and this increases the maximumspeed of the printer. The standby power consumption of this transistorcircuit is extremely low and this means that a multitude of thesecircuits can be enclosed in a small cabinet without harmfulover-heating. With this circuit by controlling the on time of the powertransistor, it is a simple matter to adjust the amount of energy appliedto the hammer-firing electromagnet. The power transistor can betriggered on by its associated transistors in response to a very lowenergy pulse and accordingly the circuit is fully compatible withexisting computing equipment normally associated with a high speedprinter of this kind. By virtue of the unique way in which the circuitoperates, the power or output transistor can be a very inexpensive onerather than one specially designed for high internal power dissipationand which at the present time would be very expensive.

A better understanding of the invention together with a fullerappreciation of its many advantages will best be gained from a study ofthe following description given in connection with the accompanyingdrawings wherein:

FIGURE 1 shows a type-hammer firing arrangement embodying features ofthe invention; and

FIGURE 2 shows typical current-voltage curves for a power transistorwith certain load lines superimposed on these curves.

The arrangement shown in FIGURE 1 comprises an electric hammer-firingcircuit, generally indicated at it which is connected by means of a pairof wires 12 to a type-hammer assembly generally indicated at 14. Thelatter, which is described in more detail in the aforesaid patent,includes an electromagnet 16 having two coils wound on a U-shaped ironcore which in turn is part of a stationary frame 24). Positioned beneaththese coils is an armature 22 which is pivoted to the frame at 24- andwhich when the coils are energized is drawn quickly upward against thecore. The normal rest position of armature 22 is determined by a setscrew 2-5, and the armature is urged into this position by a tensionspring 28.

When the electromagnet is energized the right end of the armature ismoved at high velocity upward and into contact with the rear end of atype hammer 39. This imparts an upward velocity to the hammer which thenflies as a free projectile out of contact with the armature against astrip of paper 34 and an ink ribbon 36 to force them against a desiredone of the type characters spaced around the rim of a continuouslyrotating type wheel 38. This hammer blow (which may be of the order of50 microseconds in duration) prints a letter on the paper and then thehammer quickly returns to the position shown and the paper advances tothe next line prior to printing another letter. The paper may be printedat speeds of the order of twenty lines per second. In a single machinethere may, for example, be 190 hammers arranged side-by-side in a rowperpendicular to the plane of FIGURE 1, each hammer being controlled bya separate circuit 10.

As will be appreciated, the instant of firing of hammer 38 must beprecisely controlled in time in order for it to strike a desired one ofthe characters around wheel 38 as it rotates. At the precise instant,determined by a timing or trigger pulse applied to circuit 10,electromagnet 16 is suddenly connected via leads 12 by means of a powertransistor 40 across a battery 42 (or other suitable low resistancesource) whose positive side is grounded. The current build-up thereafterin coils 18 is very rapid (being determined by their inductance). Thetime delay between the triggering of circuit and the printing of acharacter is small, but whatever delay is present, however, is uniformfrom cycle to cycle so that once the triggering pulse has been adjustedfor proper printing it can be left alone. The same triggering pulse canbe used to actuate only one hammer or all of the hammers in the machineat the same time. Some control of the instant of printing of any givenhammer without adjustment of the triggering pulse can thereafter beeffected simply by controlling the length of time during which voltagefrom battery 42 is applied to coils 18. r

In order to turn transistor 40 on and fire the hammer, the base 44 ofthe transistor is suddenly made negative and a heavy current drawn fromit. To this end base 44 is connected to the emitter 46 of a drivertransistor 48, emitter 46 being connected in series with a resistor 50to the positive side of a bias battery 52 whose negative side isconnected to the emitter of transistor 40 and the positive terminal ofbattery 42. The collector 54 of transistor 48 is connected through acurrent-controlling resistor 56 to a common lead 59. This lead isconnected through a decoupling resistor 60 to the negative side ofbattery .2 and is bypassed to ground by a large filter capacitor 61.Current drawn from base 44 of transistor 40 is returned to the emitterthrough transistor 48, resistors 56 and se, and battery 4-2. Whentransistor 48 is made conducting, it draws a heavy current from the baseof transistor 48 so that the emitter-to-collector resistance of thistransistor is extremely small. Thus, for load currents through thetransistor far in excess of normal rated value, the voltage drop acrossand power dissipated in it are still well within permissible ratings.The maximum current which flows through this transistor when on isdetermined primarily by the voltage of battery 42 and the resistance ofcoils 18. To prevent transistor 40 from being damaged by inductive surgefrom coils 18 when the transistor is turned off, the collector of thetransistor is bypassed to the negative side of battery 42 by a rectifier58 which prevents the collector from going too negative.

The base 62 of transistor 48 is connected to the collector 63 of atransistor 64, this collector being also connected to lead 59 through alow-value load resistor 66. The emitter 68 of transistor 64 is connectedto the positive side of battery 52, so that the transistor is normallybiased on. The base 70 of transistor 64 is biased through a diode 72 anda diode 74 connected in series to the positive terminal of battery 42.The junction of diodes 72 and 74 is connected through a high value biasresistor 75 to lead 59. Normally transistor 64 is turned on but when apositive pulse is applied to base 7 0 through capacitor 76 from a timingor trigger source (not shown) the transistor is turned off. This turnstransistors 48 and 40 on to fire hammer 30.

When driver transistor 48 is turned on, a positive voltage is generatedacross load resistor 56 and this is fed back over a lead 78 through acapacitor 80 and diode 72 to base 7% of transistor 64. This voltagekeeps transistor 64 turned 01f for a length of time determined by thecharging time-constant of capacitor 80 and a resistor 82 connectedbetween base '70 and lead 59. During this on time, a heavy current isdrawn from the base of transistor 40 and this insures that it willconduct with an extremely low voltage drop between emitter andcollector. Resistor 82 is made adjustable so that the on time can bevaried. Diode 72 provides a high input impedance for the positive timingpulse to transistor 64 and diode 74 acts as a DC. restorer for capacitor80. Alternatively, where a low impedance input can be tolerated, thesediodes can be omitted and capacitor 80 connected to base 7% oftransistor 64 through a coupling resistor 83, indicated by dotted lines.

To prevent the circuit from being turned on by a trigger pulse, ifdesired, transistor 64 is connected in parallel with a transistor 84which is adapted to be turned on by a negative voltage applied to itsbase through a resistor 85 or, alternatively, off by a positive voltage.When transistor 84 is on, driver transistor 48 cannot be turned on; whentransistor 84 is off, driver transistor 48 can be turned on to turntransistor 48 on.

FIGURE 2 shows curves of emitter-to-collector voltage (Eec) versuscurrent (lee) for various emitter-to-base bias currents (Ieb) andillustrates the mode of operation of transistor 48. When off, thecurrent through this transistor is substantially zero and the voltageacross it is equal the voltage of battery 42 as indicated at point A.When the emitter-to-base current of the transistor is suddenly increasedto saturation value, as indicated by point B, the internal resistance ofthe transistor drops from a value of many thousands of ohms to nearlyZero. At saturation, i.e. any value of base current along line S onwhich the current from emitter to collector is primarily determined bythe load resistance and supply voltage, the voltage drop from emitter tocollector for a germanium transistor is only a tenth of 21 volt or so.When the current flowing into coils 18 through the transistor begins tobuild up, because the coils are inductive the load line follows alongthe curve L in FIGURE 2 instead of the straight line R which illustratesa resistive load. Accord ingly, at no time in going from points A to Balong curve L is the power dissipation of the transistor, indicated bycurve P, exceeded. When the base of the transistor is again broughtpositive, the voltage across emitter and collector suddenly jumps topoint C and thereafter rectifier 58 immediately reduces the current toits value at point A. The time required for the current and voltage tochance along line BOA is only a few microseconds, however, and hencethough the normal power ratings of the transistor may be momentarilyexceeded, it is not damaged.

In an actual circuit arrangement which has been built and successfullyoperated, the following elements and circuit values were used: battery42, 27 volts; battery 52, 2 volts; coils 18, 5 ohms total; transistor40, RCA type 2N301 germanium and rated at 40 volts and 7 watts maximum;transistors 48, 64, and 84, RCA type 2N217; resistor 56, 47 ohms;resistor 50, 150 ohms; resistor 66, 1000 ohms; resistor '75, 100,000ohms; resistor 82, 33,000 ohms; resistor 83, 1000 ohms; capacitor 80,0.1 microfarad; resistor 60, 50 ohms; capacitor 61, microfarads. Thetime transistor 40 remained on in response to each trigger pulse was ofthe order of 1 to 3 milliseconds. The maximum current (Iec at point B)was about 6 amperes and reached this value near the end of each oncycle. The voltage drop across transistor 40 at point B was about 0.2volt.

The above description of the invention is intended in illustration andnot in limitation thereof. Various changes may occur to those skilled inthe art and these may be made without departing from the spirit or scopeof the invention as set forth.

I claim:

1. An improved hammer-firing arrangement for a high speed printercomprising: a hammer-firing solenoid, a low resistance source of voltageadapted to supply a relatively large pulse of current to said solenoid,a power transistor having its emitter and collector connected in serieswith said solenoid and said source, a rectifier connected across saidsolenoid and polled to prevent the voltage thereacross from reversing, adriver transistor having an input and an output, said output beingconnected to the base of said power transistor so that when said drivertransistor is off said power transistor is off and when said drivertransistor is on the base of said power transistor is biased tosaturation and said power transistor conducts a large current with asmall voltage drop across itself, a trigger transistor having an inputand an output and which is normally turned on, said output being coupledto the input of said driver transistor, and time-constant positivefeedback means for applying to the input of said trigger transistor avoltage pulse derived from the turning-on of said power transistor tokeep said power transistor on for a controlled time.

2. The arrangement as in claim 1 wherein said trigger and drivertransistors are interconnected so that when said trigger transistor ispulsed ofi by a momentary pulse,

said driver transistor is turned on and holds said power transistor onand said trigger transistor off for a precise time determined by saidfeedback means.

3. The arrangement as in claim 2 wherein said power transistor is a type2N301 or its equivalent, said battery has a voltage of about 27 volts,and said solenoid has a resistance of about 5 ohms.

4. The arrangement as in claim 1 in further combination with inhibitmeans connected to said trigger transistor to prevent it from turningoil.

5. A high speed controllable switching circuit comprising a relativelylow resistance load, a very low resistance direct voltage source, apower transistor having an input and an output said output beingconnected in series with said lead, and a plurality of transistorsconnected as a multivibrator arrangement and having an input and anoutput, said output being connected to the input of said powertransistor and adapted to drive it to saturation so that its internalresistance is very low, said input of said multivibrator arrangementbeing adapted to be connected to an external signal source, saidarrangement constituting a variable element whereby the power suppliedto said load can be variably controlled.

References Cited in the file of this patent UNITED STATES PATENTS2,864,978 Frank Dec. 16, 1 958 UNITED :STATES PATENTOFFICE D CERTIFICATEOF CORRECTION Patent N00 2 99 7 682 August 22 1961 Francis H. ShepardJr.

corrected below.

In the drawings Figa l the upper resistor 75 which is connected to thepositive side of battery 52 should be deleted.

' Signed and sealed this 27th day of March 1962.

(SEAL) Attest:

ERNEST W SWIDER DAVID L. LADD A t t e 5 ti ng Of f i C e r Commissionerof Patents UNITED STATES PATENT. OFFICE 7 CERTIFICATE OF CORRECTIONPatent No. 2 997 632 August 22, 1961 Francis H. Shepard Jr.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the drawings Fig; l, the upper resistor 75 which is connected to thepositive side of battery 52 should be deleted.

' Signed and sealed this 27th day of March 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

